Thin foils are widely used as band-pass filters, in transmission filters, and in spectroscopic applications which use irradiation wavelengths in the range of extreme ultra-violet to soft x-rays. Submicron foils, 0.1-0.2 82 m thick, have been produced in various low Z materials (low atomic number) (i.e. carbon, beryllium) by sputtering and evaporation processes. Boron films, however, have been produced by resistance heating or electron beam evaporation (Labov, S. et al., Applied Optics 24: 576 (1985). Previous efforts to prepare boron foils by sputter deposition were precluded by the availability of dense, high purity sputter targets. Typically, sputtered films exhibit superior mechanical properties and are preferred because they have fewer defects and finer morphological growth features than foils prepared by evaporative processes.
Multilayered coatings, which are used as reflective layers in x-ray optics, are typically tens to hundreds of angstroms thick. A multilayer x-ray mirror is the analog of a quarter-wave stack reflective coating with the added complication of radiation absorption in the layers. Physically, it is an alternating sequence of thin films of highly absorbing and less absorbing materials deposited on an optically smooth substrate. The layered structure is periodic and results in a large angle, resonant reflectivity which is three or four orders of magnitude greater than the simple Fresnel reflection from an unlayered surface. Reflectivity in a multilayer mirror derives from the interference of x-rays coherently scattered from the interfaces between materials of higher or lower x-ray absorption.
The quality of the multilayered optical coating is determined by the perfection of the interfaces between the layers and the uniformity of the layer dimensions. Standard methods for application of multilayer coatings use the physical vapor deposition (PVD) process of evaporation or sputtering.
The coarse layer microstructure produced and the inherent difficulty in controlling the evaporation processes adversely affects the interface perfection and layer dimensional stability, and consequently, the efficiency of the optical coating produced by such methods. The use of computer controlled sputtering processes allows the production of complex multilayer coatings with variable layer thickness and composition.